1. Field of the Invention
The present invention relates to latent curing agents for epoxy resin, and to a curable epoxy resin compositions which contain such an agent. More precisely, the present invention relates to latent, epoxy resin-curing agents that exhibit good storage stability and low-temperature rapid curability of epoxy resin compositions containing it, and relates to curable epoxy resin compositions which contain such an agent and exhibit good storage stability and low-temperature rapid curability. The present invention further relates to methods for preparing articles by curing such an epoxy resin composition and the articles thereby obtained.
2. Discussion of the Background
Recent remarkable developments in electronics have made it possible to greatly increase the degree of integration of semiconductor IC chips and have enabled mass-scale production of IC devices. With that, small-sized, thin and high-quality electronic appliances now enjoy great popularity. With the recent popularization of such electronic appliances, one important problem is how to increase the mass-scale productivity of the products and to lower the production costs thereof. Epoxy resin compositions are used as the adhesive for electronic appliances, and they require a latent, epoxy resin-curing agent. However, it is much desired to further improve the quality of the curing agent. Needless-to-say, however, the applications of epoxy resins are not limited to the electronic industry.
Epoxy resin compositions are grouped into two types. One type is a two-package composition in which the main component epoxy resin and a curing agent are mixed just before use, and the other type is a one-package composition in which the main component epoxy resin is previously mixed with a curing agent. The latter one-package composition is preferred for the reason that it is free from the problems associated with incorrect mixing of the constituent components and it is applicable to automation lines driven by machine. One-package epoxy resin compositions require a curing agent which does not react with an epoxy resin compound at room temperature but, when heated, starts to react with the epoxy compound to cure it. This type of curing agent is referred to as a latent curing agent.
Some latent curing agents for epoxy resin have been proposed. Typical examples are dicyandiamides, dibasic acid dihydrazides, boron trifluoride amine complexes, guanamines, melamines, and imidazoles. Epoxy resin compositions prepared by mixing any of dicyandiamides, melamines, or guanamines with an epoxy compound have good storage stability, however they require long term curing at a high temperature not lower than 150° C. Combining the composition with a cure accelerator to shorten the curing time is one general technique in the art. The cure accelerator is effective for shortening the curing time of epoxy resin compositions containing it, however its addition significantly reduces the storage stability of the compositions. Epoxy resin compositions containing a latent curing agent selected from a group consisting of dibasic acid dihydrazides and imidazoles can cure at relatively low temperatures, however their storage stability is not good. Boron trifluoride amine complexes are corrosive and have some negative influences on the properties of the cured products of epoxy resin compositions containing them. Under these circumstances, a latent curing agent for epoxy resin that produces epoxy resin compositions having good storage stability and low-temperature rapid curability is highly desired.
To solve the problems mentioned above, a curing agent of a dialkylamine adduct with an epoxy compound is proposed in Japanese Patent Laid-Open Nos. 155222/1981 and 100127/1982; and a curing agent of an aminoalcohol or an aminophenol adduct with an epoxy resin is proposed in Japanese Patent Laid-Open No. 53526/1984. U.S. Pat. Nos. 4,066,625 and 4,268,656 disclose a curing agent of an imidazole compound or an N-methylpiperazine with an epoxy compound added to its secondary amino group. However, the curing agents produced according to the methods disclosed are problematic in that, when they are added to epoxy resin compositions to improve the low-temperature curability of the compositions, they reduce the storage stability of the compositions and therefore shorten the shelf life or the pot life of adhesives and coating materials that contain the epoxy resin composition. Japanese Patent Laid-Open No. 296525/1991 discloses a one-package epoxy resin composition that contains a curing agent prepared through thermal reaction of an epoxide having more than one epoxy group on average in the molecule, an N,N-dialkylaminoalkylamine, a cyclic amine with an active hydrogen-containing nitrogen atom in the molecule, and a diisocyanate. However, the epoxy resin composition proposed in this publication is still unsatisfactory in view of its low-temperature curability and storage stability. Moreover, nothing is disclosed in these prior art references about to the curability of the composition at a temperature of 70° C. or lower.
Thus, there remains a need for improved latent curing agents for epoxy resin compositions which afford epoxy resin compositions which exhibit improved shelf or pot life and may be cured at low temperatures. There also remains a need for epoxy resin compositions which contain such a latent curing agent and for methods for preparing article by curing such an epoxy resin composition.